Liquid-filled transformer with foamed insulation

ABSTRACT

Electrical inductive apparatus of the type containing a magnetic core-winding assembly immersed in a liquid-insulating material. The breakdown voltage of predetermined insulating gaps in the apparatus is increased, while maintaining substantially the same dielectric constant across the gap as that of the liquidinsulating material, by disposing an open cell-insulating foam material in the predetermined gaps. The open cells of the foam material are impregnated or filled with the liquid-insulating material, which increases the breakdown voltage in volts per mil across the gap.

United States Patent XXX U mzw b y 56 e 1M 3 k W73 .W 3] 3 R S u T m m m m N m m a m m m a M A w i P m m n m 0 CS "0 m" I wm mm m 33 m h n w mamm mm m m wAAF u Tm momma E U9999 WSW HHHH .mT i nww E w, 3 e 1 Boom WW 6 2 nn 5 2233 PA 6 m r: 0 C m E 9 a a D. m .m & m C.m l. momma .H%& W 0. e m N. m w .m AHMA N. HUN 7 RBMU ABSTRACT: Electrical inductive a pparatus of the type conrsed in 'a g material. The breakdown voltage of predetertaining a magnetic core-winding assembly imme liquid-insulatin mined insulating gaps in the apparatus is increased, while maintaining substantially the same dielectric constant across the gap as that of the liquid-insulating material, by disposing an open cell-insulating foam material in the predetermined gaps. The open cells of the foam material are impregnated or filled with the 1iquid-insu1ating material, which increases the breakdown voltage in volts per mil across the gap.

HOlf 27/02 174/17, 52.6, 110.8, 58,94, 96,100

[54] LIQUID-FILLED TRANSFORMER WITH FOAMED INSULATION 6 Claims, 1 Drawing Fig.

[51] Int. [50] Fieldof 17.1, 17.1 1,

PATENTEU MAY I 8 ISYI wlmzssss l NVENTOR John C. Cr onin ATTORN LIQUID-FILLED TRANSFORMER WITH FOAMED INSULATION BACKGROUND OF THE INVENTION l. Field of the Invention The invention relates in general to electrical inductive apparatus, such as transformers, and more specifically to electrical inductive apparatus of the type having a magnetic corewinding assembly immersed in liquid-insulating means.

2. Description of the Prior Art Certain types of electrical inductive apparatus, such as transformers and reactors, include magnetic core-winding assemblies immersed in a liquid-insulating and cooling medium, such as mineral oil. The breakdown stress in volts per mil of an oil gap in such apparatus decreases, as the gap spacing is increased. Thus, it is common to divide the oil gap with a series of pressboard barriers, which reduces the dimensions of the oil gaps and thus increases the breakdown strength. In general,

may be any type of high voltage electrical power apparatus which is filled with a liquid-insulating and cooling dielectric, such as mineral oil.

More specifically, transformer 10 includes a metallic tank 12 having sidewall, bottom and top portions, with the tank 12 being filled to a level 14 with an insulating and cooling liquid dielectric, such as mineral oil. A magnetic core-winding assembly I6 is disposed in the tank 12, and immersed in the liquid dielectric. The electrical bushings associated with transformer l 10 are not shown in the FIGURE, in order to simplify the drawing. The tank 12 has a plurality of openings 18 at the level 14 of the liquid dielectric, which openings are in communication with coolers or heat exchangers (not shown) mounted outside the tank 12. The liquid dielectric flows into the heat exchangers, is colled, and collected in suitable headers, and then allowed to flow back into the tank 16 near the bottom thereof, either by natural thermal siphon effect, or

mechanical considerations determine the extent of the subdivision, resulting in a relatively few number of gaps being created. Further, the oil impregnated pressboard has a higher dielectric constant than the oil, which transfers electrical stress from the pressboard barriers or spacers to the liquidfilled gaps between the barriers. Completely solid barriers are usually avoided as it requires extreme care to avoid an oil gap in series with the solid barrier, between the electrode surfaces adjacent the gap. This is important, as the higher dielectric constant solid material would transfer electrical stress to the series oil gap which may be greater than the breakdown stress of the gap, creating corona which breaks down and degrades the solid and liquid insulation adjacent thereto, eventually causing complete failure of the insulation in this area.

Thus, it would be desirable to be able to subdivide oil gaps in electrical apparatus into a large plurality of series oil gaps, while maintaining the effective dielectric constant of the gaps substantially the same as the oil.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved electri cal inductive apparatus, such as transformers or reactors, having a magnetic core-winding assembly disposed in a tank containing a liquid-insulating and cooling dielectric, such as mineral oil. The high stress oil gaps in the apparatus, such as those between phase winding assemblies, those between the high and low voltage windings of a phase winding assembly, the areas adjacent the static plate, and the gaps between the phase winding assemblies and the tank walls, are subdivided into a large plurality of minute oil gaps by using an open cell foam material disposed in the gaps. The open cells of the foam material allow the material to become completely filled with the liquid dielectric, with the resulting liquid'filled structure having substantially the same dielectric constant as the liquid itself, due to the thin membranelike walls of the cells, and with the gap having a breakdown stress in volts per mil which is based more nearly on the cell dimensions, rather than on the dimensions of the gap in which the foam material is placed.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which the single FIGURE is a perspective view of a transformer, partially cutaway, and partially in section, constructed according to the teachings of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, the single FIGURE is a perspective view of a transformer 10 constructed according to the teachings of the invention. Transformer I0 is illustrated as being of polyphase core-form construction, but the apparatus may be single of polyphase, shell of core-form, and in general by a suitable pump. Transformer 10 may have a plurality of heat exchangers, as required by the specific application.

Magnetic core-winding assembly 16 includes a three phase magnetic core 22 having winding legs 24, 26 and 28 connected by'upper and lower yoke portions 30 and 32, respectively. Magnetic core 22 is formed of a plurality of stacks of magnetic, metallic laminations, such as grain-oriented silicon steel, with the winding legs being illustrated as having a cruciform cross-sectional configuration, in order to more effectively couple windings having round openings therein. The stacked laminations are held in assembled relation by upper and lower end frames 34 and 36, respectively.

The magnetic core-winding assembly 16 also includes phase winding assemblies 38, 40 and 42, disposed about winding legs 24, 26 and 28, respectively. Each of the phase winding assemblies includes concentrically disposed low and high voltage windings, such as low and high voltage windings 44 and 46 illustrated in phase winding assembly 38. The low and high voltage windings are separated by insulating means 48 in the highlow space between the concentrically disposed windings, and static plates, such as a static plate 50, may be disposed at the line end, or ends, of the winding structure, to more evenly distribute surge potentials across the line end pancake coil of the plurality of pancake type coils of which the high voltage winding may be formed. The high and low voltage windings are mechanically held at each end of its associated phase assembly by pressure'rings or plates, such as pressure plates 52 and 54 illustrated in assembled relation with the phase winding assembly 40. Suitable means, such as bolts 56 and 58 connected to the end frames, apply pressure at discrete points about the pressure plates of each phase winding assembly, to provide the necessary force to prevent the windings from distorting during short circuit conditions.

High voltage electrical inductive apparatus has many areas of high electrical stress, which areas must be adequately insulated to prevent corona discharges and breakdown of insulation. Examples of such high stress areas are the high-low insulation, such as insulating means 48, disposed between the high and low voltage windings of a phase winding assembly, the space between adjacent phase winding assemblies, the insulation between the static plate and pressure ring, and the space between the phase winding assemblies and the adjacent metallic tank walls.

The breakdown stress in volts per mil of an oil gap decreases as the gap spacing is increased. Therefore, the breakdown stress of an given gap can be increased by breaking the oil gap into a plurality of series oil gaps, which substantially increases the volts per mil which the gap will withstand before a breakdown occurs. The prior artdiscloses structures which divide or break up an oil gap into a plurality of series oil gaps, which structures are formed of a plurality of pressboard barriers. The number of series oil gaps which can be created is seriously limited, however, when using pressboard, due to mechanical limitations. Further, oil impregnated pressboard has a higher dielectric constant than the oil in the gaps between the pressboard layers. Since electrical stress is distributed inversely proportional to the dielectric constants of the strata of elements subjected to the stress, the stress is not evenly distributed between the electrodes which create the gap, but is concentrated in the oil-filled gaps. Attempts to eliminate oil gaps by using all solid insulation in highly stressed areas is costly, due to the extreme care which must be taken to insure that there will be no pockets of oil in series with the solid insulation, as the differences in the dielectric constants of the relatively thick solid insulation versus any small gaps of oil, will transfer stress to the small gaps in excess of the breakdown stress of the gap, resulting in corona discharges which emit radiofrequency energy, and which degrade the surrounding insulation.

These disadvantages of prior art structures may be overcome by utilizing the teachings of this invention, wherein at least the highly stressed areas of the electrical apparatus are filled with an open cell foam material having small relatively uniform cell sizes, which fill with the oil to provide a large plurality of series oil gaps. Further, the wall thickness of the foam cells is so small, the dielectric constant of a gap filled with liquid-impregnated foam material is effectively the same as the liquid itself. Thus, the stress is uniformly distributed across the total gap, with stress concentrations being eliminated. Therefore, the placement of the foam material is not as critical as the placement of pressboard barriers, whether the foam insulation is foamed in place or formed separately in suitable molds, as it will conform closely to the electrode surfaces in the high stress areas where it is applied.

The dimensions of the cells of the foam are not critical. It is important, however, that the. cells of the foam material selected beopen and interconnected. It is also important that I all of the air be removed from the cells of the foam before it is impregnated with the liquid dielectric. It may be necessary to compress flexible foam to break any cells which may not be completely open, to remove any air entrapped therein, and also to insure complete impregnation of the foam by the liquid in the electrical apparatus.

The foam material selected must be able to withstand the maximum operating temperature to which it will be subjected in the electrical apparatus. Further, it must be nonfriable and nondusting, and it must be able to withstand attack from the specific liquid used for the cooling and insulating dielectric medium. While thermosetting materials are preferable, thermoplastic foam materials having a softening temperature well above the maximum operating temperature of the apparatus would be suitable.

Flexible urethane foams have been found to be excellent in mineral oil, as they are made up of interconnecting cells separated by thin broken walls. The urethane foams have a high mechanical strength, even at low densities, and have a low compression set and a high degree of resiliency.

Tests made with urethane foams having a density of pounds per cubic foot demonstrate that the breakdown stress of an oil gap may be improved by a factor of about two, when using oil-filled open-celled foam in the gap, compared with only oil in the gap. For example, when using an electrode 'comprising a 4'-inch diameter disc having rounded edges, placed parallel to and 75inch away from a smooth plane electrode, breakdown with only mineral oil in the gap occurred as low as 35 kv., while with foam in the gap, which had the air removed therefrom, no breakdowns occurred below 100 kv. Tests of the oil between the electrodes, following removal of the foam, produced breakdown of the oil at 40 and 45 kv.

While urethane foam is preferable because it can be economically foamed in place, sprayed, or preformed in molds to special shapes, other insulating foams may be equally as suitable. For example, low density silicone foams are open celled, they have good chemical resistance, and they have a very high maximum service temperature. The open-celled phenolic foams are nonflexible and thermosetting, but they are not as desirable as they become friable at low densities. Flexible polyvinychloride foams are open celled and have a good chemical resistance, but they are thermoplastic, with a F. maximum service temperature. Since the production of foams is well known, as is the various methods of foaming the resins, it is not necessary to discuss specific foaming techniques in detail. The Handbook of Foamed Plastics, Rene J Bender, Lake Publishing Corporation, Libertyville, Illinois, 1965, describes suitable foams which may be used, as well as techniques for their production.

Transformer 10 shown in the FIGURE may advantageously utilize open cell insulating foam structures in its highly stressed areas, such as the high-low insulation 48. The high and low voltage windings may be assembled with suitable mechanical spacers, and the foam insulation applied by foaming in place, or a rectangular section or sheet of preformed foam insulation may be disposed between the windings when the phase assembly is being manufactured.

Another excellent location for the foam insulation is between the static plate, such as the static plate 50, and the adjacent pressure plate. The foam in this area may be sprayed on the pressure plate, or applied as a discrete section of insulation.

As illustrated in the FlGURE, phase barriers may be formed of large sections of open-celled foam, such as barrier 60 between phase assemblies 38 and 40, and barrier 62 between phase assemblies 40 and 42. Sections of foam insulation may also be placed between the phase winding assemblies and the adjacent walls of the tank 12, such as barrier 64 between phase assembly38 and the adjacent tank wall, and barrier 66 between phase assembly 42 and the adjacent tank wall.

While the invention has been described as being used with a mineral oil-cooling and insulating medium, it will be understood that the invention is applicable to any type of liquid coolant, such as the synthetic transformer oils. Care should be taken to insure that the foam material selected will be compatible-with the specific liquid dielectric used. For example, certain of the foams, such as the urethanes, are compatible with mineral oil, but may severely swell when being subjected to coolants which contain chlorinated products.

In summary, there has been disclosed new and improved electrical inductive apparatus in which the breakdown stress of highly stressed liquid filled gaps has been substantially increased by using liquid-impregnated open cell foam materials in the gaps, enabling insulation clearances to be reduced while eliminating corona with its degrading effects on liquid and solid insulation. Suitable foam-insulating materials may be economically produced, either separately in prefonned shapes, or in the location in which they are to be used. The open cells of the foam, filled with an insulating liquid dielectric to provide a large plurality of series liquid gaps, substantially increase the breakdown stress across a gap in volts per mil, by a factor of about two. Further, the thin membranelike walls of the cells do not cause an uneven distribution of electrical stress across the g p, as the dielectric constant of the materials in the gap is e ectively that of the liquid dielectric alone.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all mattercontained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

lclaim:

1. Electrical inductive apparatus, comprising:

a casing,

liquid-insulating means disposed in said casing,

a magnetic core-winding assembly disposed in said casing and immersed in said liquid-insulating means,

and foamed insulating means disposed between predetermined points of different electrical potential in said magnetic core-winding assembly, said foamed insulating means being of the open cell type, with the cells of the foamed insulating means being impregnated with said liquid-insulating means.

2. The electrical inductive apparatus of claim 1 wherein the foamed insulating means is a urethane foam.

. 3. The electrical inductive apparatus of claim 1 wherein the magnetic core-winding assembly is of the core-form type, having a plurality of phase winding assemblies disposed on spaced leg portions of the magnetic core, and wherein the foamed insulating means includes members disposed between adjacent phase winding assemblies.

4. The electrical inductive apparatus of claim 3 wherein the foamed insulating means includes members disposed between the phase winding assemblies and the casing.

LII 

2. The electrical inductive apparatus of claim 1 wherein the foamed insulating means is a urethane foam.
 3. The electrical inductive apparatus of claim 1 wherein the magnetic core-winding assembly is of the core-form type, having a plurality of phase winding assemblies disposed on spaced leg portions of the magnetic core, and wherein the foamed insulating means includes members disposed between adjacent phase winding assemblies.
 4. The electrical inductive apparatus of claim 3 wherein the foamed insulating means includes members disposed between the phase winding assemblies and the casing.
 5. The electrical inductive apparatus of claim 3 wherein each phase winding assembly includes concentric high and low voltage windings, and wherein the foamed insulating means includes members disposed in the high-low space between the high and low voltage windings.
 6. The electrical inductive apparatus of claim 1 wherein the foamed insulating means is a urethane foam, and the liquid-insulating means is mineral oil. 